In a tractor having a front housing which includes at its front end portion an engine flywheel and main clutch, there remains a spare space within a rear half of the front housing. Such spare space is often used for providing an additional speed change mechanism which allows, in cooperation with a speed change transmission in a transmission casing a fine control of the vehicle speed so as to adapt the same for various working purposes which are carried out using the tractor.
Japanese Patent Publication No. 63-39449 issued on Aug. 5, 1988 discloses a typical example of a transmission assembly in which such additional speed change mechanism is provided. This is done in a fashion so as to permit an easy exchange of the change mechanism according to a given design of the tractor, while remaining the speed change transmission in a transmission casing unchanged, and so as to permit an easy assembly of the additional speed change mechanism into the front housing.
In the tractor shown in this prior art literature, the front housing is fashioned to include an open rear end and an intermediate wall which is located in front of the additional speed change mechanism. Front and rear transmission casings fastened together are provided in which the front casing includes an intermediate wall, located in front of a main speed change mechanism, and a rear wall located behind the main speed change mechanism. A bearing support plate or frame is disposed within a front end portion of the front transmission casing and is secured to the rear of the front housing. A primary shaft and speed change shaft, between which the additional speed change mechanism performs a speed-change transmission, are supported respectively by the intermediate wall of the front housing and by the bearing support frame. A drive shaft, and second speed change shaft between which the main speed change mechanism performs a speed-change transmission, are supported respectively by the intermediate wall of the front transmission casing and by the rear wall of this casing. The speed change shaft of additional speed change mechanism and the drive shaft of main speed change mechanism are coupled with each other at a space in front of the intermediate wall in the front transmission casing.
In assembling the additional speed change mechanism employed in the prior art transmission assembly set forth above, the primary and speed change shafts are supported at their front end portions by the intermediate wall in the front housing under a condition where the front transmission casing is separated from the front housing and where the bearing support frame has not been secured yet to the rear of front housing. Then gears and other elements of the additional change mechanism are mounted from the rear end opening of front housing onto the primary and change shafts. Rear end portions of these shafts are then supported by securing the bearing support frame to the rear of the front housing. The front housing and front transmission casing are then assembled and fastened together, there being a connection between the speed change shaft of additional speed change mechanism and the drive shaft of main speed change mechanism by means of a coupling member disposed between these shafts. It is thus seen that the additional speed change mechanism can be assembled with ease. However, attention is paid only to an easy assembly of the additional speed change mechanism and, therefore, this prior art structure involves problems which will be detailed hereinafter.
First, while space in front of the intermediate wall in the front transmission casing is used for providing the bearing support frame and for providing a connection between the speed change shaft of additional speed change mechanism and the drive shaft of main speed change mechanism, the main speed change mechanism is disposed between the intermediate and rear walls of front transmission casing by using these walls as supports for the drive and speed change shafts of the main speed change mechanism. Such structure will make assembly of the main speed change mechanism very troublesome. That is, gears and other elements of the main speed change mechanism must be put into the front transmission casing through an opening to be formed in the top wall or one of the side walls of the casing and, then, the drive and speed change shafts must be set between the intermediate and rear walls by passing it through one of these walls under a condition where the gears and other elements referred to above are kept in their proper positions. This assembly procedure is very hard and expensive.
Further, when design of the tractor or its transmission assembly is changed so that it comprises the same transmission mechanism within the transmission casing but now includes no additional speed change mechanism within the front housing, a large dead space will be resulted within the transmission casing because the space in front of the intermediate wall of the casing is used only for providing a connection between shafts.